1. Field
This invention relates to advanced microelectronic (semiconductor) devices and methods for fabricating the same, and in particular, to microelectronic devices containing a region of optically active material that permits the device to be closed with a pulse of light of one wavelength; and then opened with a pulse of light of a second wavelength.
2. Background of the Technology
The circuit shown in FIG. 1 is widely used in diverse applications where the conversion of one voltage or current (usually DC) to a three phase AC voltage or current (or vice versa) is required [1]. Examples include motor drives for electric vehicles, industrial motors used in factories, and utility power conditioning systems such a static volt-ampere-reactive (SVAR) compensators and rectifiers and invertors used for high-voltage DC electric power transmission, two-switch and four-switch versions of this circuit (the “half bridge” and the “full bridge,” respectively) are common in power supply applications used throughout the defense and civilian electronics industry.
The circuit has six semiconductor switches that can be constructed in many forms, including the bipolar junction transistor (BJT), the metal-oxide-semiconductor field effect transistor (MOSFET), the insulated gate bipolar transistor (IGBT), the static induction transistor (SIT), thyristors of the silicon controlled rectifier (SCR) type, the gate-turn-off (GTO) type, or the static induction type [2]. Many other variations of the above can be found in the prior art.
The basic circuit building block found in FIG. 1 is the two-switch half-bridge phase leg (see FIG. 2). FIG. 2 also shows two disadvantages of this prior art. The first is commonly known as the “high-side gate driver” problem in which the upper switch S1 is electrically controlled by gate driver circuitry whose common connection is the load, and thus a floating gate drive is required. This introduces greater complexity and cost into the final system. The second problem is the possible introduction of incorrect gate signals which could cause improper operation of the half-bridge, possibly causing a failure to occur in either the circuit or the load. The source of these incorrect gate signals is commonly called “electromagnetic interference” or EMI. EMI can come from many sources and can effect all applications. But in military related systems, there is the additional threat of intentionally introduced EMI from enemy action. EMI can effect the operation of any and all switches in the circuit, including the low-side switch S2 in FIG. 2.
Optically controlled circuits represent one remedy to both the high-side gate driver problem and the EMI problem. FIG. 3 illustrates another embodiment of the prior art that partially remedies the problem. The introduction of an additional circuit in the gate driver is called an optical receiver that allows a fiber optic connection between a central processor and any of the switches in the circuit of FIG. 3. The fiber optic link is generally much less vulnerable to EMI, if not immune. Unfortunately, the problem of providing isolated electrical power to the receiver and the gate driver remains. And the gate driver circuitry is still potentially vulnerable to EMI. The former problem is especially troublesome whenever a long string of devices are connected in series to multiply the total blocking voltage of the stack, as is often the case in electric utility equipment.
A typical response is to eliminate, if possible, the gate driver circuitry all together. The use of optically active switches is one solution. FIG. 4 reveals additional prior art in which optically active devices, usually optically triggered thyristors, are used because they do not need a gate driver to be switched on or “closed.” Generally, optical radiation of a characteristic wavelength generated by a laser (but other sources of optical energy can be used) is conducted by suitable means (usually fiber optic cable) to the switch. Electron-hole pairs are generated in the portion of the switch that is illuminated such that the device switches into conduction [3]. The principal limitation is that the switch cannot usually be switched off with light, which accounts for the popularity of thyristors because they can generally be switched off by the external circuit through a process known as commutation. This limits the optically triggered thyristor, by far the most commonly used optically active switch used in power electronics, to applications where circuit commutation is possible; however, in many applications commutation is not an option which severely limits the application of the prior art in optically active switches.
Optically active BJTs, also known as phototransistors, are commonly used in the microelectronics industry in devices such as “optical isolators” (or “opto-isolators” for short) and light detectors of various kinds and applications. Phototransistors are rarely used in circuits like FIG. 1, but in principal they could be. Phototransistors of the prior art are a variation on the optically triggered thyristor in that electron-hole pairs are generated by a light source with a photon energy that exceeds the bandgap energy of the semiconductor used in the transistor. The base of the BJT is usually chosen to be the optically active medium. An advantage of the optically active BJT is that conduction through the transistor will continue only for as long as the light shines on the base of the BJT. When the light is removed, the BJT will stop conducting current and in due course the switch will turn off or “open.” The problem is that the delay prior to switch off is generally determined by the physics of minority carrier storage in the base of the BJT which is generally slow for BJTs that have good optical gain [4]. The phototransistor can be made faster by introducing impurities that result in a short minority carrier lifetime (MCL) but this negatively impacts the optical gain. In most applications, the optical energy required to initiate and sustain conduction is an important figure of merit, where less is much better.
Similar problems arise in the development of semiconductor switches intended to control large amounts of transient power, known as pulsed power generators. These systems are generally found in defense and medical applications. Very fast switching is demanded by such applications [5], which has made semiconductor device development by the pulsed power technical community rather distinct from that developed for applications in the conventional power electronics community. In the pulsed power community switches that close when illuminated by laser light and then open when the laser light is removed with a time constant characteristic of the material are said to operate in the “linear mode” [6]. Linear-mode switches can be characterized as “light-sustained” bulk photoconductive closing and opening switches. Such switches are similar in this respect to the phototransistor except that they are simpler in construction, often consisting of little more than a block of semiconductor, such as silicon or gallium arsenide, with a metal contact on either end to form Ohmic contacts for connecting the switch to the external circuit; and their size is typically much larger which reflects their completely different application [7]. However, the disadvantageous trade-off in laser energy for switching speed remains the same [8].
An alternative to the “light sustained” photoconductive switch is taught by Schoenbach et al. in U.S. Pat. No. 4,825,061 [9], which reveals a bulk photoconductive device in which a laser pulse of one wavelength stimulates persistent photoconductivity which continues for up to many microseconds after the laser pulse of nanosecond duration has terminated [10]; and which can be terminated on demand by application of a second “quenching” laser pulse of longer wavelength [11]. Schoenbach et al. in '061 takes advantage of the optical quenching effect which was known by 1960 to be particularly strong in Gallium Arsenide doped with copper [12]. The physics of infrared optical quenching in photosensitive semiconductors like copper-doped GaAs and CdS, the fundamental basis of '061, were adequately understood by 1965 [13]. The teaching of Schoenbach et al. in '061 is limited to the use of these effects in a bulk photoconductive switch whose embodiment is described generally in [7] and [9] and is illustrated in FIG. 5. A substantial literature, e.g. [14] and [15], reveals that the teaching can be practically realized by a photoconductive switch intended for circuits like that shown in FIG. 6 and that are generally utilized in pulsed power applications, for example, as taught by Stoudt et al. in U.S. Pat. No. 5,864,166 [16]. All demonstrations of practical working devices have been limited to the bulk photoconductive switch taught by Schoenbach et al. in '061 and fabricated with the same core process of compensating silicon-doped GaAs with copper (GaAs:Si:Cu) by thermal diffusion to make a bulk semi-insulating material [17]. Indeed, no other practical teaching is contained in '061.
The advantage of the GaAs:Si:Cu photoconductive switch, as compared to the pulsed power switching prior art, is that it has high photoconductive gain in a material with short minority carrier lifetime, thus offering a much lower consumption of laser power to applications requiring current pulses with fast rise and fall times and a long and/or continuously variable duty ratio. However, as reported in [14] only relatively low average electric fields of the order of 3 kV/cm can be controlled in GaAs:Si:Cu bulk photoconductive switches because of a fundamental instability that leads to current filamentation [18], so to block large voltages and to conduct large currents, an extremely large active area is required with respect to the conventional semiconductor devices used in the power electronics industry. Therefore, prohibitively large laser energy is required to apply the switch to power electronics applications. An additional disadvantage is that GaAs is generally a poor choice for power electronics due, among other reasons, to its low thermal conductivity. Schoenbach et al. does not teach an embodiment that can be practically applied to a better choice of semiconductor for power electronics, such as silicon carbide.